Uranium is a naturally occurring element which is comprised of approximately 0.7%.sup.235 U and 99.3%.sup.238 U. .sup.235 U is used to produce Nuclear Energy, while .sup.238 U is not. Because of the low percentage of .sup.235 U found in naturally occurring uranium, naturally occurring uranium must be enriched in order to obtain sufficient amounts of .sup.235 U to which will support nuclear fission. This enrichment process, aside from producing high concentrations of .sup.235 U, produces huge amounts of depleted uranium hexafluoride (UF.sub.6) by-product which is a very hazardous compound posing a serious health threat since depleted uranium metal is radioactive and expensive to produce, it is used in limited quantities for highly specific applications. Accordingly, alternative uses are needed in order to avoid having to dispose of the UF.sub.6 at great expense by storing it indefinitely. One solution to reducing the large stores of UF.sub.6 is to convert the UF.sub.6 into uranium oxide, e.g. UO.sub.2 or U.sub.3 O.sub.8.
One use for uranium oxide is to add it to the concrete which is used to build bunkers in which radioactive waste is stored in order to provide high density shielding for the stored high level radioactive waste. Uranium oxide possesses outstanding radioactive shielding properties, and when added to concrete in the form of aggregate makes a very effective low cost shielding material.
There are many methods which can be used to convert UF.sub.6 into uranium oxide. Many of these methods also produce valuable by-products. However, methods which are currently used to convert the UF.sub.6 into uranium oxide taint these valuable by-products with radioactive uranium, rendering the by-products commercially unusable, requiring additional processing to remove the radioactivity, as well as additional contaminants, or storing the by-products as low level radioactive waste.
The most common method for producing uranium oxide includes reacting UF.sub.6 gas with steam (H.sub.2 O) and hydrogen (H.sub.2). This produces uranium oxide and hydrogen fluoride gas (HF). However, because the products and reactants are mixed in the gas phase, this HF gas, although having economic value, is contaminated by unreacted uranium thus reducing its value as discussed above. Moreover, it is highly diluted, due to the steam, further reducing its value.
Another method for producing uranium oxide reacts UF.sub.6 with H.sub.2 to produce uranium tetrafluoride (UF.sub.4) and HF gas. The UF.sub.4 is then reacted with steam to produce a uranium oxide, UO.sub.2 or U.sub.3 O.sub.8, and HF gas. However, the reverse reaction is so strong that tremendous amounts of steam must be used to avoid a reverse reaction. This not only requires a large amount of energy to produce the steam, but again produces a highly diluted hydrogen fluoride product that has little commercial value, requiring further processing to obtain a valuable product. Moreover, complete conversion to uranium oxide is nearly impossible thereby degrading the uranium oxide's suitability for making concrete and thus the value of the uranium oxide produced.
Accordingly, the major drawbacks of the presently preferred methods are that the HF is diluted and contaminated with some amount of uranium making it commercially unusable. Thus, while HF has some economic value, the uranium contamination reduces this value and further provides yet another storage dilemma, as encountered with all radioactive waste, or additional processing to purify the HF.
Moreover, these methods are very expensive. Thus, an economical way to convert UF.sub.6 to uranium oxide is needed in order to make use of the large quantities of radioactive waste such as UF.sub.6 and produce commercially valuable by-products which are not radioactive and require no additional processing.
One solution to reducing the large stores of UF.sub.6 is to reduce UF.sub.6 to UF.sub.4 and convert the UF.sub.4 into SiF.sub.4, and an oxide of uranium, e.g. UO.sub.2 or U.sub.3 O.sub.8.
Silicon tetrafluoride is widely used in the manufacturer of semi-conductor chips, pyrogenic silica, and other industrially important chemicals.
Silicon tetrafluoride can be produced in several ways all of which are based on reacting silica (SiO.sub.2) with either hydrofluoric acid (HF) or fluorosilicic acid (H.sub.2 SiF.sub.6). Thus to produce SiF.sub.4 from SiO.sub.2, the production of either hydrofluoric or fluorosilicic acid intermediate is required.
Current processes to produce SiF.sub.4 in, varying degrees of purity, include the reaction of silica with hydrogen fluoride gas according to the reaction: EQU SiO.sub.2 (s)+4HF(g).fwdarw.SiF.sub.4 (g)+2H.sub.2 O
See U.S. Pat. No. 4,382,071.
Purity of the SiF.sub.4 is dependent on the source of the silica and hydrogen fluoride reagents. The reaction is typically carried out at 25.degree.-55.degree. C. in concentrated sulfuric acid (&gt;80% H.sub.2 SO.sub.4) in order to diminish the reverse reaction through capture of the product H.sub.2 O. This process also uses large amounts of anhydrous HF which raises concerns for corrosion, safety and environmental management.
Production of SiF.sub.4 from fluorosilicic acid can be accomplished according to the reaction: EQU SiO.sub.2 (s)+2H.sub.2 SiF.sub.6 (aq).fwdarw.3SiF.sub.4 (g)+2H.sub.2 O
See U.S. Pat. No. 4,470,959. This reaction is also carried out in concentrated sulfuric acid (&gt;80% H.sub.2 SO.sub.4), but usually at a slightly higher temperature, between 25.degree.-95.degree. C.
It is also possible to produce SiF.sub.4 directly from fluorosilicic acid by thermal decomposition: EQU H.sub.2 SiF.sub.6 (aq).fwdarw.SiF.sub.4 (g)+2HF(g)
However, typical input fluorosilicic acid (20-30% aqueous) comes from fertilizer and phosphoric acid/super phosphate manufacturing waste tails. The fluorosilicic acid is generally low grade containing many impurities such as phosphorous, nitrogen and sulfur, all of which are detrimental to producing high purity SiF.sub.4.
Yet, another multi-step process for producing SiF.sub.4 utilizes the reaction of fluorosilicic acid with sodium fluoride and silicon dioxide according to the reaction: EQU 2H.sub.2 SiF.sub.6 (aq)+6NaF+SiO.sub.2 .fwdarw.3Na.sub.2 SiF.sub.6 (s)+2H.sub.2 O
followed by thermal treatment of the fluorosilicate salt at 600.degree. C. to release SiF.sub.4 according to the reaction: EQU Na.sub.2 SiF.sub.6 .fwdarw.SiF.sub.4 (g)+2NaF
See U.S. Pat. No. 4,615,872.
As with the processes discussed above, this introduces impurities through use of low grade fluorosilicic acid diminishing the purity of the silicon tetrafluoride produced.